My invention relates to the inspection and testing of solid conductors. More specifically, it relates to a device and method for measuring conduction current through a sample portion of the surface of a solid conductor that is in contact with a liquid conductor, such as the solid electrolyte of a liquid-solid-liquid battery.
Storage batteries having solid electrolytes are well known. The cells of such batteries have a liquid anode, a solid electrolyte, and a liquid cathode. Typical anodes for these batteries are composed of liquid sodium or lithium or potassium. Typical cathodes are molten sulfur mixed with sulphur salts. Typical electrolytes are solid ceramics, made by high temperature sintering of a mix of stable chemical compounds. The most commonly preferred electrolytes are special forms of alumina, i.e., aluminum oxide crystals, doped heavily with an oxide of the conduction ion. These electrolytes conduct ionic current but act as insulators to electron flow.
To date, one potentially promising solid electrolyte battery has been the sodium-sulfur battery. This battery has an anode of molten sodium and a cathode of molten sulfur in solution with sodium sulfide. The most common electrolyte for such a battery is a sodium-beta"-alumina electrolyte. To maintain the anode and the cathode in a liquid phase, the sodium-sulfur battery typically operates at between 315 degrees C. and 350 degrees C.
A significant advantage of the alumina-electrolyte battery in general, and the sodium-sulfur battery in particular, is its specific energy capacity, i.e., its electrical storage capacity per pound. The sodium-sulfur battery, for example, theoretically could have a specific energy capacity that is eleven times that of the lead-acid battery. In actual practice, the specific energy capacity of such batteries has been reportedly measured at about 7.4 times that of the lead-acid battery. Despite this impressive advantage and extensive research in the field, a great deal of difficulty has been encountered in developing a battery having a reliable solid electrolyte. The solid electrolyte develops cracks, and no battery designer has been able to make a battery with a commercially acceptable life. These cracks generally start on the sodium (anode) side of the battery. Sodium dendrites fill the cracks and ultimately reach the sulfur (cathode) side of the battery, internally short circuiting the battery which results in failure. Although various theories have been advanced as to the cause of the cracking in the electrolyte, the exact cause of the cracking remains unknown.
Typically, examination of failed electrolyte samples is performed using a scanning electron microscope or an atomic force microscope. Each of these devices can be used to map the topography of the surface of the sample in order to detect cracks. However, these devices suffer from a number of testing drawbacks. Because flaws consisting of potentially destructive metal dendrite growth locations are characterized by high conductivity, a device capable of mapping the conductivity of the electrolyte in detecting and examining the cause of electrolyte failure is useful. However, neither the scanning electron microscope nor the atomic force microscope measure data that can be used to map the conductivity of the electrolyte. Likewise, neither of these devices can measure impedance characteristics of the electrolyte, which would be useful in failure analysis.
In addition, both the scanning electron microscope and the atomic force microscope require removing the solid electrolyte from the cell in order to inspect its surface. Consequently, the devices cannot be used to observe surface anomalies as they develop over time or to observe such anomalies while the battery is subjected to an operational electrical load or charging current.
Moreover, the atomic force microscope cannot provide data regarding anomalies that are below the surface of the electrolyte. Although the scanning electron microscope can provide some data regarding subsurface anomalies, electron microscope examination of the electrolyte is a destructive examination. To conduct such an examination, the electrode sample must first be prepared by depositing a conductor on its surface so that the electrons from the beam can be dissipated to prevent charge build-up. This preparation procedure contaminates the sample and renders it unusable as an electrolyte.
For the foregoing reasons, there is a need for an apparatus and method suitable for measuring and mapping the conductivity of the surface of a solid conductor that is in contact with a liquid conductor, such as a solid electrolyte of a liquid-solid-liquid battery. Yet another need for such an apparatus and method is that it be suitable for doing so without contaminating the electrolyte.
A further need for such an apparatus and method is that it be suitable for making such measurements while there is electric conduction across the solid-liquid interface of the conductors, such as when a solid-liquid-solid battery is subjected to an operational electrical load or charging current.
A further need for such an apparatus and method is that it be suitable for measuring and mapping the impedance characteristics of the surface of the solid conductor. An additional need for such an apparatus and method is that it be suitable for providing data regarding anomalies below the surface of the conductor.